The present invention relates to a fluid dynamic bearing unit that drives a fluid such as a lubricant to generate a dynamic pressure, and to cause relative contactless rotation between a shaft and a retaining member thereof.
Fluid dynamic bearing units and motors including the same are employed in various rotational mechanisms that are continuously put into service over a long period of time, and hence required to have a long product life and reliability for a long-term use, such as a DC motor for driving a hard disk. One of such motors having a fluid dynamic bearing unit is disclosed in JP-A No. H10-318253, which will be reviewed below referring to FIG. 14. FIG. 14 is a fragmentary cross-sectional view of the fluid dynamic bearing unit according to the cited document, only showing the right relevant parts thereof for the purpose of description.
In FIG. 14, a column-shaped shaft 101 is fixed to a base plate 102, which is configured in common with a base member for a hard disk unit, for example. The shaft 101 is provided with a disk-shaped flange 126 fixed to an upper end portion thereof, according to the orientation of FIG. 14. The shaft 101 is inserted into a bearing bore 130a of a sleeve 130. The sleeve 130 is rotatably supported by the shaft 101. A hub 108 having a rotor magnet 112 is attached to the sleeve 130. The hub 108 includes a disk seat 110, on which a recording and reproducing disk, such as a magnetic disk (not shown) is placed. The base plate 102 is provided with a stator 114 located so as to oppose the rotor magnet 112, and to thus give a rotating force to the hub 108. Above the flange 126, a ring-shaped cap 138 is provided.
The bearing bore 130a of the sleeve 130 is provided with a herringbone-shaped radial dynamic pressure groove (not shown) formed on the inner circumferential surface thereof, which constitutes a radial fluid dynamic bearing section 144. The flange 126 is provided with a herringbone-shaped thrust dynamic pressure groove (not shown) formed on both faces thereof, which respectively constitutes a thrust fluid dynamic bearing sections 154 and 156. The flange 126 includes two circulation holes 176 oriented substantially perpendicular to the central axis of the shaft 101, with approximately 180 degrees of central angle. The inner circumferential end portions of the circulation holes 176 are respectively communicating with a through hole 174 formed on an outer circumferential surface of the shaft 101 in an axial direction. Both open ends of the through holes 174 communicate with a gap between the shaft 101 and the bearing bore 130a. The circulation holes 176 and the through holes 174 achieve communication with a gap 156a between the upper face of the flange 126 and the cap 138, a gap 154a between the lower face of the flange 126 and the sleeve 130 and a gap 180a between the outer circumferential surface 180 of the flange 126 and the sleeve 130. A lubricating fluid such as an oil (hereinafter, simply referred to as oil) is filled in the gap defined by the shaft 101, the flange 126 and the bearing bore 130a of the sleeve 130.
When injecting the oil into the gap, bubbles are inevitably mixed in the oil. Among such bubbles, description is made as to the bubble that may be present in the gaps 154a and 156a on and under the flange 126. When the sleeve 130 rotates, the oil is subjected to a pressure and a centrifugal force originating from a pumping effect, in the thrust fluid dynamic bearing sections 154 and 156. Though the flange 126 is provided with the thrust dynamic pressure grooves at the upper face and the lower face, normally it is difficult to form the thrust dynamic pressure grooves such that the dynamic pressures on the upper face and the lower face of the flange 126 are accurately balanced in a radial direction. Since an accurate balance of the dynamic pressures is not obtained, the pumping pressures applied to the oil by the thrust fluid dynamic bearing sections 154 and 156 are not balanced either, in a radial direction. Accordingly, the oil pressure in the gap 180a and the oil pressure in the through holes 174 become different, thereby causing the oil to flow from a higher-pressure region toward a lower-pressure region. When the oil flows, for example in a direction indicated by the arrows 190 and 191, the oil circulates through the gap 180a, the gaps 154a and 156a, the through holes 174 and the circulation holes 176. When the oil circulates as above, the bubble in the oil is separated from the oil by a ring-shaped recess 162 provided along an inner circumferential portion of the cap 138, when the oil flows into the through hole 174 from the gap 156a. The separated oil is discharged to ambient air through a gap between a minor diameter portion 101D of the shaft 101 and the cap 138. In this way, the bubbles that have been mixed into the oil at the time of injecting are separated by turns, while circulating the oil utilizing the inevitable unbalance of the dynamic pressures in a radial direction generated by the thrust dynamic pressure grooves, until finally the oil becomes clear of bubble.
In the above-mentioned conventional fluid dynamic bearing unit, the circulation hole 176 of the flange 126 is formed by a perforating process such as drilling. Likewise, the through hole 174 is formed by a machining process to cut away a portion of the surface of the shaft 101, thus to form a flat portion.
When the flange 126 is thicker than approximately 3 mm, it is relatively easy to perform the drilling process to form the circulation hole 176 through the flange 126. However, in the case of a small fluid dynamic bearing unit having a flange of 2 mm or less in thick, an extremely slender drill tip, for example 0.5 mm in diameter, has to be employed, which is too fragile in the drilling tool, therefore, the drilling process becomes difficult. Besides, since the drilling process requires a certain time, it is difficult to reduce the processing time, and it is difficult to reduce the processing cost.
Further, the drilling process produces fine metal powders as a result of cutting a metal material. Removal of the metal powder requires a meticulous cleaning process. However the metal powder may still remain unremoved within the circulation hole in the flange, even after the cleaning process. In the event that the metal powder comes off and intrudes into the narrow gap between the shaft and the sleeve during the operation of the fluid dynamic bearing unit, the rotation is disturbed, and it is not possible to rotate in worst case, thus resulting in a failure of the fluid dynamic bearing unit.